In an internal combustion engine, generally, deposit such as the carbon deposit being residue of fuel or lubricant oil is accumulated inside the combustion chamber with the prolonged operation. For instance, the deposit is accumulated on the piston surface, the fuel injection valve, and the like. As a result, the compression ratio increases, causing NOX, CO, HC, etc in the exhaust gas to show an increasing trend. FIG. 12 is a diagram of a relation between deposition of the deposit inside the combustion chamber (travel distance) and NOX in the exhaust gas in an internal combustion engine provided for an automobile. As shown in FIG. 12, the amount of NOX increases with the deposit deposition, and NOX in the exhaust gas decreases by cleaning inside the cylinder and removing the deposit. The same trend can be observed for CO and HC in the exhaust gas.
Conventionally, a variety of methods have been employed for combustion control of the internal combustion engine and monitoring of the combustion state. For instance, as disclosed in Patent Documents 1 and 2, there are combustion control methods of monitoring the cylinder internal pressure of the internal combustion engine. These methods merely detect the combustion state based on the pressure waveform inside the cylinder and correct the fuel supply amount, the fuel supply timing, and the injection timing based on the detected result. Thus, the existence of the deposit is not detected. Therefore, even if some abnormality has occurred for a certain cylinder and the deposit is markedly increased, such abnormality cannot be detected promptly, which may lead to troubles such as damage to the internal combustion engine.
Further, in the case where tuning is performed upon installation for operation of the internal combustion engine, the exhaust gas property needs to be set with a margin in anticipation of the change due to aging, because the deposit deposition state can be detected only by opening the engine (or checking with a fiberscope while the engine is stopped). Accordingly, it is not always possible to provide an operation setting with high efficiency.
Further, conventionally, maintenance for removing the deposit is automatically set to be performed at a certain interval of the operation period. Thus, unnecessary maintenance is performed for an internal combustion engine with the operation conditions that produce a small amount of deposit even for the engine of the same type, causing extra maintenance cost. In contrast, for an engine with the operation conditions that produce a large amount of deposit (such as an engine often performing low-load operation), there is a possibility that the timing for removing the deposit is delayed and the exhaust gas exceeds the regulation value.
On the other hand, Patent Document 3 discloses a means to determine whether there is any deposit accumulated inside the cylinder. In the presence of deposit, the fuel is absorbed by the deposit. Thus, this determination means focuses on the increased number of injection by the injectors compared to the normal state being required before a rich signal is outputted by the oxygen level sensor provided for the air discharge pipe. That is, the injector of a certain cylinder is stopped for a certain period of time and then restarted, and then existence of deposit is determined based on the number of injection performed by the injector before a rich signal is outputted from the oxygen level sensor therein.
Further, patent Document 4 discloses a means for estimating the deposit deposition amount accumulated mainly on the fuel injection valve and the like. This estimation means estimates the deposit deposition amount from a correlation map between the deposit deposition amount and the operation state amount (such as net mean effective pressure and rotation speed) obtained in advance by experiment and the like.